A polarizing plate has been generally used in a structure in which a protective film is laminated on one surface or both surfaces of a polarizer using an adhesive, the polarizer being formed of a polyvinyl alcohol (hereinafter, referred to as ‘PVA’)-based resin dyed with a dichroic dye or iodine for durability and water resistance. In this case, triacetyl cellulose (TAC)-based films have been frequently used as the protective film in view of excellent optical transparency or moisture permeability.
Meanwhile, as liquid crystal display devices have been recently developed into mobile devices such as laptop-type personal computers, mobile phones, and car navigation systems, thinning in thickness and reduction in weight have been required for a polarizing plate constituting an image display device. However, in a polarizing plate in which TAC films and the like are laminated as the protective film as described above, it is difficult to set the thickness of the protective film to 20 μm or less from the viewpoint of handleability during operation or durability performance, so that there is limitation in thinning in thickness and reduction in weight.
In order to solve the problems as described above, there has been proposed a technology of forming a transparent thin film layer by providing a protective film on only one surface of a polarizer and painting an active energy-ray curable composition on the opposite surface, or painting the active energy-ray curable composition on both surfaces of the polarizer without the protective film. Meanwhile, the active energy-ray curable composition proposed up to now may be divided into a radical-curable composition and a cationic curable composition in accordance with the curing method. In this case, when a transparent thin film layer is formed by using a cationic curable composition, the transparent thin film layer has an advantage of excellent adhesion with a polarizer, but has many disadvantages of manufacturing processes due to a slow curing rate and a low degree of curing.
In order to solve these problems of the cationic curable composition, there has been proposed a technology of forming a transparent thin film layer by using a radical curable composition including an acrylic compound as a main component. However, since the radical curable composition including an acrylic compound as a main component has a faster curing rate than that of the cationic curable composition, but the protective layer has a low glass transition temperature after radical curing, there is a problem in that a polarizing plate manufactured by using the composition has poor heat resistance reliability.
Further, the radical curable composition is required to have high elastic strength because cracks are generated on a PVA polarizer when thermal shock is evaluated due to structural characteristics of the polarizing plate. In general, in order to increase elastic strength of the composition, a polymer constituting the composition needs to be crystallized or have a high degree of crosslinking, and the degree of crosslinking of the composition has been increased by using a multifunctional acrylate-based compound such as TMPTA, PETA, and DPHA as an actual method. However, there is a problem in that the multifunctional acrylate is recklessly used to significantly decrease adhesive strength with a PVA polarizer, thereby incurring peeling.
Therefore, in order to solve the aforementioned problems, there is need for a new polarizing plate having a high glass transition temperature and elastic strength, and excellent adhesive strength between a protective layer and a polarizer, while having a protective layer which may be formed by curing radicals.